How to make square responsive divs on flexbox?

Question

css

Task:
1. There is a block.
2. Its width must be equal to the width of the parent.
3. Its height must be equal to its width for any.
4. When changing the width of the parent, the block should change proportionally (width and height should be equal to each other).
5. It is desirable that it be on flexbox.

Answer 1

codepen.io/AndrewHaze/pen/LxaQeE